Electric current in semiconductors

1. The thermistor has a resistance of 50kΏ at a temperature of 20⁰C and at a temperature of 25⁰C its resistance decreases to 42.5kΏ. Determine the average value of the temperature coefficient of resistance in this temperature interval.

2.The average value of the temperature coefficient of resistance of a thermistor is α = –0.05K^–1. By how much must the temperature of the thermistor increase so that its resistance decreases to half?

3.A thermistor connected in an electric circuit is heated by the flame of a gas burner. The dependence of the thermistor's resistance on temperature is given by a table. What is the temperature of the thermistor if the reading on the voltmeter is 3V and on the ammeter 15mA?

4.An NPN transistor is connected in an electric circuit. If the base current changes from 0.2mA to 0.3mA, the collector current changes from 5mA to 20mA, at a constant collector voltage of 4.5V. Determine the current gain factor β of the transistor.

5.A semiconductor sample cut from a silicon single crystal has the shape of a cuboid with a length of 5mm and a base area of 4mm². On two opposite bases with an area of 4mm² metallic contacts are formed. If a voltage of 4.5V is applied between the contacts, a current of 1.2mA flows through the sample at a temperature of 20⁰C. After heating the sample to a temperature of 300⁰C the current increases to 2.16mA. What is the resistivity of silicon at temperatures of 20⁰C and 300⁰C? Also determine the temperature coefficient of electrical resistance α.

6. The voltage on the thermistor has a constant value U = 4.5 V. At temperature t₁ = 20⁰C a current
I₁ = 9 mA flows through the thermistor, at temperature t₂ = 22⁰C the current is I₂ = 10 mA. Determine the values of the thermistor resistance at these temperatures and also the temperature coefficient of resistance α. 

7.What is the current gain factor β of a transistor if a change of base current from I_B1 = 0.1 A to I_B2 = 0.2 A causes a change of collector current from I_C1 = 6 A to I_C2 = 22 A?

8.A transistor operating in a common-emitter configuration has a base current I_B1 = 30 μA and collector current I_C1 = 2 mA. Determine the collector current I_C2 at a base current I_B2 = 100 μA if the current gain factor is β = 60.

9.In electrical engineering practice, germanium is considered pure if there is at most 1 impurity atom per one billion (10⁹) Ge atoms. Determine the mass of completely pure germanium that could be contaminated by one gram of iron. The relative atomic mass of germanium is 72.59 and of iron 55.837. Express the result in tonnes.

10.A transistor operating in a common-emitter connection conducts currents I_B = 20 μA and I_C = 2 mA. The voltage between the base and emitter is U₁ = 0.7 V, and the voltage between the collector and emitter is U₂ = 20 V. Calculate the power in both sources and compare them.

11.A resistor with resistance R and a thermistor are connected in series in a circuit. At temperature t₁ the milliammeter shows current I₁. If the thermistor is placed into hot oil, the milliammeter shows current I₂. Determine by what factor the resistance of the thermistor changes after immersion in the oil. The voltage source has voltage U. Solve the problem for these values:

12.A sample of doped silicon in the shape of a cube with edge 1 cm contains 10¹⁴ phosphorus atoms. Two opposite faces of the cube are metal-coated and the sample is connected across a voltage of 4.5 V. A current of 108 mA flows through it. Calculate:

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